Conventionally, a tire-pressure monitor system (Tire Pressure Monitoring System: TPMS) which informs a driver of tire-pressure information has been known. The tire-pressure monitor system comprises a tire-pressure sensor unit (hereafter, will be referred to as a sensor unit) which detects a tire pressure for each wheel, and is configured to transmit tire-pressure information as a radio signal from this sensor unit and to receive the radio signal with a tire-pressure notification device disposed on the vehicle body side to acquire the tire-pressure information. When judged that the tire pressure has been decreased based on the received tire-pressure information, the tire-pressure notification device displays that on an annunciator and notifies it to the driver.
Since the sensor unit does not usually comprise a reception function, it cannot be configured to transmit the tire-pressure information in response to a demand from the tire-pressure notification device. For this reason, in the tire-pressure monitor system, the tire-pressure information is unilaterally transmitted from the sensor unit in a predetermined period. The sensor unit has a battery built therein and transmits the radio signal by using this battery as a power supply. For this reason, it is required that the battery life of the sensor unit should be secured to the same extent as the life time of the vehicle.
Since the sensor unit is attached to an air supply valve portion, it is small and the size of the battery to be built in is also restricted. For this reason, it is necessary to transmit a radio signal so that battery consumption may be suppressed as much as possible. Although the battery life can be lengthened when the transmission period (interval) of the radio signal is lengthened, the acquisition of the tire-pressure information by the tire-pressure notification device will get delayed.
For instance, a tire-pressure monitor system which transmits tire-pressure information in a constant period only when judged that a vehicle is running is proposed in the Patent Document 1 (PTL1). A sensor unit disposed in this tire-pressure monitor system comprises an acceleration sensor in addition to a pneumatic sensor, and it judges that the vehicle is running (wheel is rotating) when acceleration in the direction of centrifugal force detected by this acceleration sensor is larger than a threshold value, and transmits tire-pressure information periodically. Thereby, the battery consumption during the vehicle is stopping can be suppressed.